The advantage of using quartz in electronics is now well known for obtaining oscillators that are stable and for making filters. The use of this material is expanding very rapidly at present and billions of quartz-based devices are now manufactured each year in the world. The advantage of quartz analogues is that they have frequency stability and Q-factors similar to those obtained using quartz while also possessing piezoelectric properties that are greater, thus making it possible to make filters with much larger relative bandwidth, and also oscillators that are capable of being frequency offset much than is possible using quartz.
Berlinite, a monocrystal of the alpha phase of aluminum phosphate (ALPO.sub.4), is the quartz isostructural compound that has been most developed. It has the advantage of enabling filters to be obtained with twice the bandwidth and oscillators that can be offset twice as far as those that use quartz. Berlinite is the material most suitable for making the intermediate frequency filters for the future pan-European digital radio telephone that requires a passband of 200 kHz to 70 MHz. With berlinite, as with quartz, an important characteristic is crystals of large size and of very good quality suitable for making devices that use said material under conditions that are economically favorable. Other crystallographic analogues of quartz are being developed (GaPO.sub.4, AlAsO.sub.4, FePO.sub.4, etc.), and they have the same advantages as berlinite, sometimes to an even greater extent.
For quartz, as for analogues thereof, the common crystal orientations that are the most advantageous for applications are the AT, BT, SC, IT, and ST cuts. To obtain plates having said orientations and crystal morphologies that are the most advantageous for applications from the cost point of view, it is most common to use the following:
for quartz, seeds that are oriented normally to the optical axis z, that are long in the y direction, and that are considerably shorter along the x axis. In some cases, it is also possible to use seeds having the same orientation but that are long in the x direction and short along the y axis. Seeds whose thickness is normal to the various natural faces of the rhombohedron may also be of significant advantage.
for berlinite, parallelepiped-shaped seeds that are oriented normally to the electrical axis x, that are long in the y axis direction, and that are of small height and thickness in the optical axis direction z. It is also possible to use seeds that are oriented normally to the z axis.
In certain special cases, that are less common, other types of seed may be used, and this applies in particular to quartz when it is necessary either to obtain plates that are very large and of a shape that is not compatible with using a Z seed crystal, or to obtain plates having special crystal orientations more cheaply.
Hydrothermal growth methods allow a large quantity of crystals to be obtained in a single operation and thus under cost conditions that are highly favorable. They often lead to quite good crystal quality. For materials having phase transitions between ambient temperature and their melting point, such methods are among the few methods that can be used.
Several hydrothermal growth methods are known for quartz. They use alkaline solutions of sodium carbonate or of sodium hydroxide. In such solvents, quartz solubility is direct (and increases with temperature) such that the mother body placed in the hot bottom portion (at about 673 K) of the autoclave dissolves in the form of silicate. The solution is transported by convection towards the cold portion (temperature about 50.degree. C. lower than the hot portion) where it desaturates by depositing quartz on the seeds. The conditions that give rise to a pure material (low OH and Al content) are conditions where temperatures and pressures are high and speeds are moderate (a few tenths of a millimeter per day and per face). These conditions are also conditions that give rise to low dislocation density.
In practice, the seeds are generally parallelepiped-shaped plates having sides of very different dimensions. Their length L is generally chosen to lie in the crystal direction that has the slowest rate of growth (y for quartz and berlinite) and their thickness (generally in the range 0.5 to a few mm) lies in the useful growth direction that is to be enhanced, while their width l (often called "seed height" and equal to about 10 mm to 50 mm for quartz) is usually chosen as a function of the applications intended, and in this case this parameter (like the growth times) can be used to optimize the size and the quantity of seeds obtained.
Recent work has shown that the presence of extensive faults (and in particular dislocations) has a large effect on the operation of highest performance piezoelectric devices.
In addition, the industry is making use of collective manufacturing techniques for said devices by chemically cutting or etching wafer of quartz or of its analogues. The yield of the corresponding operations is usually severely affected by the presence of dislocations in the material used, which give rise to localized etching anomalies ("etch pits", "etch channels").
Various methods have been proposed to improve not only the impurity concentration (such as point defects associated with hydrogen, lithium, sodium, etc.), but also to reduce the increase in the number of dislocations in quartz crystal, in particular by varying the intrinsic conditions of crystal growth (solution, temperature, pressure, seed orientation, . . . ).
However such improvements have been too small or have required the use of experimental conditions that are too severe.
Two techniques are known for obtaining quartz crystals having low dislocation density. Under such circumstances, growth is usually performed using Z orientation seeds since the morphology of the resulting crystals is favorable to obtaining those crystal orientations that are used most often (AT, BT, ST, SC, etc.) and the Z growth zones then used have qualities that are advantageous from the impurity concentration point of view.
The technique most widely used relies on the use of seeds that possess such quality themselves and that have been obtained by a small number of successive growths made on a seed taken from a natural quartz crystal (which material has extremely low concentration dislocations). In the prior art, the successive growths are formed along the optical axis, and dislocations existing in the seed propagate during growth so that their density cannot decrease. In practice density increases quickly depending on the quality of growth conditions, because of new dislocations appearing at the seed-growth interface, or in the growth itself (together with the existence of inclusions that are difficult to avoid altogether).
Using that technique on an industrial scale to obtain crystals having a very low concentration of dislocations therefore requires frequent regeneration of seed quality by using natural quartz. Economically speaking, this makes the method very expensive (large natural crystals are expensive).
For other materials obtained using the hydrothermal technique and for which large-sized natural crystals of great perfection do not exist, the above method cannot be applied.
In 1981, following fundamental work on the influence of sector localization of the seed by A. Zarka et al. (A. Zarka, Liu Lin, X. Buisson, "The influence of sector localization of the seed on the crystal quality of synthetic quartz", published in J. Cryst. Growth, Vol. 54, pp. 384-398 (1981)) propose a novel technique making it possible for the first time to genuinely reduce the density of dislocations without making massive use of expensive natural crystals. That technique of growing synthetic quartz is illustrated in accompanying FIG. 1 to 3.
FIG. 1 is a section perpendicular to the y direction through a crystal of synthetic quartz made from a seed whose thickness extends normally to the usual growth direction, i.e. the z direction.
FIG. 2 shows the result obtained after growth from the seed G1 shown in FIG. 1.
FIG. 3 shows the crystal obtained after growth from the seed G2 shown in FIG. 2.
In the prior art, the seeds G2 (FIG. 2 ) are Z plates taken from the X or S growth zones of a synthetic crystal made conventionally using a Z type seed G1 (FIG. 1) from the above-mentioned growth method. It can be seen in FIG. 2 that the dislocations 11 that existed in the seed G1 extend into the zone Z. Additional dislocations may exist in the Z zones, due, for example, to solid inclusions trapped at the beginning of growth. Dislocation density therefore increases with successive generations.
In that case, few dislocations 12 (FIG. 3) existing in the seed G2 obtained in this way extend in the Z zones of the new growth, and it is possible to obtain crystals having a very low dislocation density. The quality of this method has been checked by X-ray topographical investigation. Furthermore, the method has the advantage of taking seeds from zones of growth that are not used for making devices, and which are in any case cut off during initial cutting.
Using said technique, the best zone for cutting out the seed G2 is the zone X (FIG. 2). In quartz crystals, X zones are either small in height (crystals obtained under conditions that give rise to high rates of growth in the z direction in an NaOH or an Na.sub.2 CO.sub.3 medium), or else they are small in width (crystals obtained using small rates of growth under conditions that lead to high intrinsic Q-factor). As a result, in that method, either it is difficult to obtain seeds of great height, or else it is possible to obtain only a very small number of seeds per crystal.
With reference to FIG. 2, it should be observed that only a very small number of G2 type seeds can be obtained per crystal.
Thus, the technical problem to be solved by the present invention is to provide a method of obtaining a crystal by performing crystal growth in the liquid phase on a seed, which method makes it possible to solve the problem of dislocation density while avoiding systematic use of natural crystals, and secondly to provide a significant number of seeds, which is an important point when producing crystals having low dislocation density on an industrial scale.